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Effects of bracken [Pteridium aquilinum (L.) Kuhn] control treatments on the dynamics of bracken litter and its nutrients, and the potential consequences for species diversity

*R H Marrs, K Galtress, #Tong, C. , +S J Blackbird, T J Heyes and M G Le Duc

Applied Vegetation Dynamics Laboratory School of Biological Science University of Liverpool PO Box 147 Liverpool L69 3GS and +Department of Earth & Ocean Sciences University of Liverpool 4 Brownlow Street Liverpool L69 3GP #Present address, Department of Ecology and Environmental Sciences Inner Mongolia University 010021Huhhot Inner Mongolia China *Corresponding author: calluna@liv.ac.uk 37 http://appliedvegetationdynamics.co.uk 38

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Tel +44 (0) 151 794 4752 Fax +44 (0) 151 794 4940 October 2003

Summary 1

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Bracken (Pteridium aquilinum (L. Kuhn) is an invasive, clonal fern which often reduces species diversity and

conservation interest. It can produce a deep litter layer that impedes the establishment of new species. There

have been few studies on bracken litter and its dynamics, especially during bracken control. This study

examined the effects of bracken control treatments applied over seven years on bracken litter distribution,

nutrient compartmentation and litter turnover and their relationship with developing species diversity.

Bracken litter was > 2000 g m-2 in untreated plots, and control treatments reduced this quantity to varying

degrees. The distribution of nutrients in the soil-plant profile suggested that (i) cations were rapidly leached

from the standing fronds, C, N, P and Ca amounts were greater in the soil profile with the greatest organic

matter, whereas K and N were greater in the mineral fractions., and (iii) the rhizomes are a major store of

nutrients, especially Ca and Mg, with 30% of the total amount in the profile found there.

Cutting twice/year was the most successful treatment in reducing bracken litter. This treatment was also

associated with a significant increase in bracken litter turnover and mass of non-bracken vegetation. Other

treatments also reduced bracken litter mass, and accelerated litter turnover and development of non-bracken

vegetation, but not to the same extent. There was a significant amount of nutrients released by the cutting

twice/yr treatment; in absolute terms large amounts of C and N were released, but when expressed as a

percentage of the total amount in the system, between 19-26% of the total Ca and Mg in the system was

released from the rhizomes. Although, some of these released nutrients were taken up by the developing

vegetation, there was a net loss to the system as a result of bracken treatment. This release has implication in

terms of reducing C stocks and also potential loss of Ca and Mg, which will enhance acidification.

Species diversity was greater where bracken litter had been reduced, but there was an important interaction

with sheep grazing. Where bracken litter mass was low, there was increased diversity in the sheep grazing

treatment, whereas only Deschampsia flexuosa was associated with the ungrazed treatment.

The study provided further evidence that litter is an important above-ground component of the bracken

habitat which must be considered in the restoration of sites with a dense bracken cover. Cutting twice/yr was

the most effective treatment in reducing bracken and litter cover, but it also released considerable quantities of

nutrients into the system, greater than the uptake by developing vegetation and their litter. The consequences of

this are a release of C to the atmosphere and cations (especially Ca and Mg) available for leaching, although

they may also be taken up by soil microbes. Irrespective, there is a potential dilemma between controlling a pest

species, which has evolved to sequester nutrients and potential long-term degradation.

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Key-words: moorland, litter turnover, nutrient compartmentation, decomposition, restoration, land

management

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Bracken (Pteridium aquilinum (L.) Kuhn)1 is an invasive, clonal fern which covers large tracts of land in the

UK (Pakeman and Marrs 1992). It is a particular problem in upland and marginal areas (Pakeman et al. 2000a).

Its spread over recent decades has been partly blamed on a combination of (i) a decline in harvesting for soap,

livestock bedding and thatch and (ii) a general reduction in its management (Pakeman et al. 2000b). Bracken is

a problem for many land users. It can have a substantial commercial impact through obstructing agricultural

and forestry practices, it is poisonous and carcinogenic to browsing livestock (Marrs et al. 2000), and it presents

a risk to human health by harbouring ticks, which act as vectors for Lyme disease. Although records indicate

that bracken may not have increased dramatically in total abundance compared to historical values, it has

replaced substantial areas of plant communities regarded as having greater conservation and amenity value

(Pakeman et al. 2000a). It poses a threat to biotopes of international importance that are subject to Biodiversity

Action Planning, such as lowland heaths and heather moorland, of which the UK contains ca. 20% and 70% of

the European stock respectively (Thompson and Usher 1991, Dolman and Land 1995). The canopy of bracken

fronds, which emerges in spring, shades out most competitors and the thick litter layer, which develops

underneath, hampers the growth of other species (Marrs et al. 2000). The uncompacted dead fronds and petioles

compete with other species for space, while older compacted litter appears to hinder their germination (Paterson

et al., 2000). In woodlands, it may also delay tree regeneration (Pakeman et al. 2000a, Marrs et al. 2000).

Moreover, it has been predicted that the area of land dominated by bracken will increase under the reduced

stocking densities encouraged under Agri-Environment schemes, and under global warming (Pakeman et al.,

2000a).

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Bracken’s success as a weed depends to a large extent on the size of its perennial rhizome network, which

acts as a large store of carbohydrates, nutrients, and perennating buds for summer frond production (Watt 1976;

Marrs et al. 1992, 1993, 1998a). These large reserves make bracken eradication from areas of extensive

invasion almost impossible with available control methods (Marrs et al.,, 1998b). The major treatments

currently used are application of the selective herbicide asulam (product Asulox, active ingredient, methyl (4-

aminobenzenesulphonyl) carbamate) and mechanical treatment (usually cutting), or combinations of the two.

Currently, in the UK asulam is applied by aerial spraying on between 5000 - 8000 ha each year (Pakeman et al.

000a).

A great deal of experimental work has been done to test the efficacy of different combinations of these

control treatments, although much of this has concentrated on their effects on frond cover and biomass and on

1 P. aquilinum is referred to as bracken throughout; nomenclature follows Stace (1997) for higher plants and Corley & Hill (1981) for bryophytes.

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the relationship between these two parameters and species diversity. There has been very little research into the

effect of bracken control on litter dynamics and its relationship between bracken frond production and

developing understorey vegetation, even though bracken litter disturbance or removal is known to increase the

establishment rate of other species (Lowday and Marrs 1992,Mitchell et al. 1999). This paucity of information

was highlighted as the reason for the very poor predictive estimates of bracken litter produced by a computer

model (REBRA) designed to predict the outcome of bracken control treatments at Cavenham Heath in Suffolk

(Paterson et al. 2000). Where decomposition rates have been measured they are slow; estimates of the

exponential decay rates for a Cumbrian site indicated that it would take between 11 and 13 years for almost

complete decay to occur (Ling-Zhi Chen and Lindley 1981).

The lack of information on litter distribution and dynamics is surprising since it is known that litter

abundance affects frond development (Watt 1956, 1969, 1970), rhizome distribution (Watt 1976) and

vegetation development after control (Lowday and Marrs 1992). Litter depth can have both positive and

negative effects on frond development. Deep litter tends to delay frond development, presumably as a result of

reduced surface and soil temperatures, but it can also help protect emerging fronds from early frosts emergence

(Watt 1956, 1969, 1970). The depth of the rhizomes under the ground can also be affected by litter depth. The

rhizomes rise towards the surface and in some cases actually grow through the litter layer (Watt, 1976), and

where the litter is removed in such situations the rhizomes near the soil surface are more readily affected by

frost (Snow and Marrs 1997). Removal of the litter layer has also been shown to promote the establishment of

new vegetation in previously species-poor vegetation under dense bracken (Lowday and Marrs 1992).

The mode of action of bracken control treatments will have both direct and indirect effects on bracken litter,

and there are important differences in the way that the treatments act. Mechanical treatment (especially cutting)

has the most direct effect because, although the cutting process severs the fronds just above ground level, it is

not selective, and any litter present will also be shredded and compacted to a greater or lesser extent. This

treatment accelerates the comminution phase of decomposition (Swift et al. 1979). However, there are also

indirect effects. Cutting is applied continuously for several years to reduce the rhizomes’ carbohydrate and

nutrient reserves and is targeted to coincide with the maximum transfer of carbohydrates and nutrients from the

rhizomes to the developing fronds, and to precede the fronds redirecting resources back into the rhizomes (late-

July in the UK). As this process is continued, the frond production, and hence inputs to the litter, should be

reduced each year (Marrs et al. 1998b). On the other hand, asulam is usually applied once in late July-early

August, when the fronds are relatively unlignified, so that herbicide enters the tissues easily, and when the

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rhizomes have become the major sink for photosynthate and nutrients (Paterson et al. 2000). The asulam is

translocated in the phloem to meristematic areas in the rhizomes (ie short shoots with frond-bearing buds),

where it prevents further frond development. Usually, there is a very good kill of frond-producing buds on the

rhizome, resulting in a 95-99% reduction in frond production in the year after spraying, although there can be

rapid recovery of frond biomass if follow-up treatments are not applied (Marrs et al. 1998b). Thus, after

spraying there should be a reduction in inputs to the litter to almost zero in the year after spraying, followed by

an increase. No information is available on whether litter turnover changes after these relatively drastic

treatments are applied.

This aim of this study was to quantify the effects of a selected suite of bracken control treatments on the

mass, compartmentation, turnover and nutrient content of bracken litter in a long-term experiment designed to

investigate integrated bracken control and moorland regeneration methods at Hordron Edge in Derbyshire, and

relate the litter dynamics to the species diversity of the developing moorland vegetation

Methods

The experiment was set up in 1993 to test a range of bracken control and vegetation restoration treatments at

Hordron Edge in Derbyshire (National grid reference, 4213 3870; Longitude and Latitude, 1041’W, 53023’N).

This experiment was set up in 1993 and uses a randomised block design with split-split plots in three replicate

blocks in a bracken patch that was wholly covered with dense bracken litter ca. 30 cm deep (Le Duc et al.

2000). The main-plot treatments were: (1) control (untreated), (2) cutting (flail cutter trailed by ATV) once per

year (cut once/yr), (3) cutting twice per year (cut twice/yr), (4) asulam application in first year only (asulam),

(5) asulam application in first year followed by single cut in second (asulam + cut), (6) cutting in first year

followed by asulam in second (cut + asulam). The split-plot treatments were: (1) no sheep exclosure, sheep

grazed at approximately 0.5 ha-1 (Pakeman et al. 2000b), and (2) sheep exclosure, ungrazed.

Each year a range of measures was made: (1) summer bracken frond biomass (June and August), (2)

bracken litter cover and depth (June) and (3) number, identity and % cover of species in the vegetation (June)

(detailed methods are available in Le Duc et al. 2000). For this part of the analysis floristic data collected in

2000, ie the summer immediately preceding the litter sampling were used. Three additional diversity measures

were calculated from the floristic data for each split-plot: (1) Shannon-Weiner index, (2) equitability and (3)

Simpson’s Index. As all measures gave similar results, only species number and Shannon-Weiner index are

presented here.

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Field sampling of litter, vegetation and soils 1

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On 12th October 2000, seven years after the experiment was started, a 0.25 m2 quadrat was located at pre-

selected random co-ordinates in each plot (10m x 10m), and the following samples taken in order: (1) the

standing, senescent fronds were cut at ground level (denoted the uncompacted bracken litter fraction); (2) the

remaining vegetation (biomass + necromass including compacted bracken litter) was cut at ground level; (3)

two 15-cm deep soil cores were taken using a Dutch bulb auger (5.08cm diameter). These cores were then

divided into three sub samples; fibrous organic material (A00 horizon), dark, amorphous organic material (A0

horizon) and mineral material (A1 horizon). All samples were returned to the laboratory. The vegetation

samples were sub-sampled randomly and sorted into bracken litter (denoted the compacted bracken litter

fraction) and non-bracken plant vegetation\litter. These sub-samples and all other samples were then oven dried

at 850C for 48 h and weighed. All data were converted to g m-2 for each fraction.

Calculation of bracken litter turnover rates

A litter turnover index (K) was calculated as the current year’s litter input (L, g m-2) divided by the total above-

ground (i.e. standing + compacted) crop of litter (XL, g m-2) (Swift et al. 1979). Because of limitations of the

available data for calculating L, two estimates of L (L1 and L2) were made, resulting in two turnover rates (K1 =

L1/XL, K2 = L2/XL) for each quadrat. For control and non-cut treatments L1 was estimated using August frond

biomass data. For the cut once/yr treatment L1 was estimated using June + August frond biomass data and for

the cut twice/yr treatment L1 was estimated as June + August frond biomass + the mass of senescent fronds in

October. Thus, L1 is an estimate of the maximum possible bracken litter input during 2000. L2 was estimated for

control and non-cut treated split-plots using the mass of senescent fronds in October. For the cut once/yr

treatment L2 was estimated using the mass of senescent fronds in October + the June frond biomass, and for the

cut twice/yr treatment, L2 was estimated using the mass of senescent fronds in October + June + August. Thus,

L2 is an estimate of the minimum possible bracken litter input during 2000.

Estimation of nutrient concentrations

The soil and litter samples were ground to a fine powder using a Kika-Labortechnik A10 mill. C and N were

estimated using a Carlo Erba Instruments NC2500 elemental analyser. Samples were then digested using the

hydrogen peroxide: sulphuric acid digestion reagent recommended by Allen (1989). P was estimated

colorimetrically by ammonium molybdate-stannous chloride methods; K and Na were estimated by emission

spectrophotometry and Ca and Mg by absorption spectrophotometry (Allen 1989). Data were expressed on a

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mass basis (g per unit mass of tissue\soil) or on an area basis (g m-2). C:N and C:P ratios were calculated for

some of the nutrient pools.

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Data analysis

Initially, data were analysed by analysis of variance using a randomised block split-plot experimental design

(PROC ANOVA, SAS 1989). Percentage cover data were arcsin-transformed (arcsine(√x%/100) and all other

data except the diversity indices were transformed using 1n(Y+1) (Sokal and Rohlf 1995). Kendal rank

correlation coefficients (PROC CORR, SAS 1989) were calculated between (i) diversity, biomass and litter

variables, (ii) bracken litter and biomass variables and (iii) turnover rates and both nutrient pools and derived

ratios. Regressions, linear and quadratic regressions, were fitted to some relationships using PROC REG (SAS,

1989).

Multivariate analysis using CANOCO for WINDOWS (ter Braak and Šmilauer 1998) was employed

to determine which factors were influencing species composition. This was done using two datasets: Analysis 1

used the floristic data collected in 2000, the summer preceding the sampling, and Analysis 2 used all the

floristic data collected between 1994 and 2002. In both analyses, a Detrended Correspondence Analysis (DCA)

was used initially to measure gradient lengths; in both cases the gradient length was < 2.5, and accordingly the

linear Redundancy Analysis (RDA) was selected for all further analyses (ter Braak and Šmilauer 1998). Species

data were 1n(Y+1) transformed, the downweighting option for rare species was not used, and significance was

tested using a Monte Carlo test with 999 permutations. The resultant biplots were produced using

CANODRAW (ter Braak and Šmilauer 1998).

In Analysis 1 the Forward Selection procedure was used to select those environmental variables (eg

bracken litter depth and grazing) that accounted for a significant proportion of the explained variation (P <

0.05). A final RDA was then done including just those selected environmental variables. The eigenvalues were

of the four axes were: λ1 = 0.215, λ2= 0.075, λ3 = 0.194 and λ4 = 0.097. The first axis and the model were both

significant (P < 0.001) with F-values of 9.031 and 6.731 respectively.

In Analysis 2, a RDA was done with bracken litter cover alone as the constrained variable, and with all

other experimental treatments and time factored out as covariables. The eigenvalues were of the four axes were:

λ1 = 0.109, λ2= 0.111, λ3 = 0.090 and λ4 = 0.068. The model was significant (P < 0.002) with an F-value of

145.28.

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Results

Effects of treatment on litter mass, turnover, frond production and non-bracken vegetation For all bracken variables and biomass of non-bracken vegetation there were significant differences because of

the bracken control main-plot treatments, but no significant effect of exclosure. There was only one significant

main-treatment x sub-treatment interaction, involving the August frond dry mass.

Bracken control treatment had a significant effect on the mass of uncompacted, compacted and total bracken

litter (P < 0.01) (Table 1). Cutting and asulam-treatment plots had significantly less of bracken litter than

untreated plots; plots with the combined treatments were not significantly different from untreated plots (Table

1). Cutting twice/yr significantly reduced uncompacted and total bracken litter mass compared to all other

treatments, and for mass of compacted bracken litter, cutting twice/yr also produced the greatest reductions,

these being significantly lower than for all other treatments except cutting once/yr.

Treatment in most cases increased the mass of non-bracken vegetation relative to untreated plots (P < 0.05)

but asulam + cutting was an exception. Cutting twice/yr resulted in the greatest increase (Table 1). Cutting,

especially twice/yr significantly increased the bracken litter turnover (P < 0.001). For K1 (based on maximum

possible litter input), cutting once/yr and twice/yr significantly increased turnover compared to untreated plots

and asulam treatment. The combination treatments were intermediate (Table 1). For K2 (based on minimum

possible litter input), only cutting twice/yr resulted in a significantly faster, being between three to twenty times

faster than all other treatments.

Bracken control treatments also reduced summer frond mass: the untreated and asulam+cut treatments had

the greatest mass, other asulam treatments were intermediate, and the cutting treatments had the lowest mass

(Table 1).

Effects of treatment on species diversity and the relationship between diversity, biomass and litter turnover

No significant differences were found between bracken control main-plot treatments and their interaction with

the grazing sub-treatments on total species number/plot or Shannon-Weiner index. However, grazing

significantly increased diversity compared to the ungrazed treatment: grazed mean = 9.3 species/plot versus

ungrazed mean = 6.3 species/plot; LSD (P<0.05) = 2.38, F1,12 = 7.22; P < 0.05 and for the Shannon Weiner

index a grazing mean = 2.05 versus ungrazed mean = 1.37; LSD (P<0.05) = 0.459, F1,12 = 10.36, P < 0.01.

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Both species number and Shannon Weiner index showed a significant negative relationship with all bracken

litter biomass variables (P < 0.05). An example is illustrated in Fig. 1a.. Non-bracken biomass showed no

significant relationship with any measure of diversity. The litter turnover rates gave conflicting results, with K1

showing no significant relationship with any of the species diversity measures but K2 showing a significant

positive relationship (P < 0.05) with total number of species (Fig. 1b)

Each of the bracken litter variables showed significant positive relationships with each other and with

summer frond variables (rk > +0.41, P < 0.001), and significant negative relationships with turnover rates and

with non-bracken vegetation mass (rk > -0.42, P < 0.001). Non-bracken vegetation mass also showed significant

negative relationships with summer frond mass (rk > -0.35, P < 0.01) but significant positive relationships with

turnover indices (rk > +0.55, P < 0.001) (Table 2).

Relationship between treatments, environmental variables and community composition

The biplot for analysis 1 (Fig. 2a), based solely on the study year revealed a diversity gradient along axis 1,

corresponding positively with increasing bracken litter depth. As litter depth decreased, there was a transition

from a bracken-dominated community at the positive end of axis 1 through to an acid heath/grassland at the

negative end. Grazing was extremely influential on axis 2, with grazed plots having a greater positive score and

ungrazed ones having a negative one. Where bracken litter mass was low and sheep grazing occurred, there was

a relatively diverse mixture of grasses, herbs and moss species typical of an acid heath/grassland community,

whereas ungrazed plots were dominated by Deschampsia flexuosa. Where bracken litter mass was high, grazing

had a lesser impact, but on ungrazed plots typical heath species (Erica tetralix and Vaccinium vitis-idaea) were

found, possibly because these species were grazed in preference to the less palatable bracken where sheep were

present. The inclusion of Agrostis castellana in the species data set indicated immigration from adjacent plots

where it had been sown as a nurse; it is unlikely to have been present in the seedbank before 1993 (Le Duc et

al., 2000).

The biplot for analysis (Fig. 2b) based 10 years data, where all environmental variables have been factored

out other than bracken litter cover, shows more or less the same pattern, with almost all species being placed at

the negative end of the bracken litter cover gradient. There was a strong gradient on axis 2, reflecting the

influence of grazing. Here the only species positively associated with bracken litter was the liverwort

Lophocolea bidentata. Galium saxatile was positioned at the negative end of the bracken litter gradient, but this

may reflect a response to increased light rather than litter per se.

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Effects of treatments on nutrient distribution, fluxes and relationship with turnover 1 2 3

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The nutrient concentrations for both the litters and the soils (Table 3) were within the ranges quoted by Allen

(1989) for similar materials. When expressed on a mass basis there were differences in the concentrations of

the different elements. The compacted bracken litter had marginally greater concentrations of N, C and P than

the standing litter, but the standing litter had much greater concentrations of K, Mg and Na (~x2), and

especially Ca (x13) than the compacted litter. This result suggests that as the litter changes from standing to

compacted form, N, C and P amounts increase perhaps as a result of decomposition processes, but the cations

are reduced as a result of leaching. The non-bracken vegetation had slightly greater concentrations of N

(slightly) and P (x2), approximately similar concentrations of K, and lower concentrations of C, Ca, Mg and Na

than the standing bracken. The concentrations within the soil profile separated elements into two groups; C, N,

P and Ca reduced through the profile indicating that they are associated with the levels of organic material

present; K, Mg and Na increased through the profile indicating that there are large amounts associated with the

mineral matrix.

When these date were expressed on an area basis the amounts of nutrients in the litter pools were in the

order compacted bracken litter > non-bracken litter > standing bracken litter, and the soil pools were A1 > A0 >

A00, reflecting the different mass of the fractions per unit area. The total amount of nutrients within the

untreated ecosystem (n=6, exclosure treatments pooled) were estimated in g m-2 (15 cm depth) as: C =

10278±1151; N = 455±45; P = 75±7; K = 724±67; Ca = 60±7; Mg = 68±6; and for Na = 81±6.

There were no significant effects of applied treatment (P < 0.05) on nutrient concentration in any soil or

litter fraction when expressed on a mass basis. On an area basis, however, there was again no differences

between the soil fractions, alone or combined, but there were significant effects of bracken control treatment on

the nutrient content in the three litter pools (Table4), there were no exclosure effects. When the litter data were

tested in pooled form, only the bracken litter (standing + compacted) showed significant effects of bracken

treatment.

The effects of the bracken treatments varied for each element in terms of significance and the same

responses were not found for the impact of the respective treatments, ie as the effect was not solely a function

of the mass of the fraction. For the two bracken litter components the greatest amount of all elements were

usually in the untreated plots, although for some elements the asulam+cut or cut+asulam treatments were the

greatest. Usually, the order was untreated > asulam+cut > cut + asulam, cut once/yr > Cut twice/yr. In all cases

the Cut twice /yr treatment was the lowest. These results were essentially mirrored in the non-bracken

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vegetation; the untreated vegetation had the lowest amounts of all nutrients, and there were greater amounts in

the treated plots generally increasing through to the cut 2/yr treatment (Table 4).

Significant differences were found between the two types of bracken rhizome for C, Ca and Mg (Table 3),

with the short shoots having a greater C and Ca concentration and the long shoots a greater Mg concentration

when expressed on a mass basis, but all elements had a significantly greater amount in the long shoots

compared to the short shoots when expressed on an area basis. A substantial fraction of the total nutrient pool

was found within the rhizomes for all elements except K (Table 3). This increased in order N < C < K < P <

Mg and Ca, and for Ca and Mg, 30% of the total mass of nutrients within the ecosystem was found in the

rhizomes.

The effects of bracken control treatments had a significant effect on the mass of long shoots and the N

concentration (mass basis) in short shoots, but no significant effect on mass of short shoots or total rhizome

mass (Table 5a). When expressed on an area basis bracken control treatment reduced the N, C, Mg, Ca and K

content, but not P or Na (Table 5b). The general treatment responses were in order untreated > combined

treatment > asulam > Cut once/yr > Cut twice/yr.

The correlation between the two turnover indices and the nutrient concentrations confirmed that the mass of

the litter fractions were the main factors controlling bracken litter turnover. There were no significant

correlations with any soil fraction (mass or area) or for the concentration of any litter fraction when the data

were expressed on a mass basis. There were significant correlations between both rate constants and the mass of

the standing litter (r = -0.647, P <0.001 with K1, r = -0.740, P <0.001 with K2), compacted litter (r = -0.542, P

<0.001 with K1, r = -0.419, P <0.001 with K2), and the non-bracken litter (r = 0.551, P <0.001 with K1, r =

0.560, P <0.001 with K2). When expressed on an area basis, there were significant correlations between the

turnover rates and the element content of each of the litter fractions (standing litter – all negative, r > -0.308, P

<0.02; compacted litter – all negative, r > -0.0.625, P < -0.001; non-bracken litter – all positive, r > 0.376, P<

0.01). There were also no significant correlations between either turnover index and C:N and C:P ratios.

The fluxes of nutrients released or taken up were estimated by calculating the difference between (1) the

amount of nutrient in the rhizomes, bracken litter and non-bracken litter pools in treated plots from (2) the

amounts in comparable untreated plots (Table 6). Overall here was in general a release of most elements except

Na from the rhizomes, and the amount released increased with increasing success of treatment, with the Cut

twice/yr treatment releasing the greatest amount. For this treatment the amount released by element was in

order C > N > K >Mg > Ca > P, but as a percentage of the amount present in the ecosystem it was Mg > Ca > P

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> K > N > C. For Ca and Mg between 19-26% of the total amount in the system was released during the 7

years of cutting twice/yr.

In contrast the fluxes between litter pools were substantial only for C and N with a release from the bracken

litter and uptake by the non-bracken litter. Again the magnitude was dependent on treatment success with the

Cut twice/yr treatment releasing most C and N from bracken litter and taking up most in the non-bracken litter.

Although some of this release could be accounted for by uptake by the developing non-bracken vegetation, this

did not account for all of it (Table 6). .

The implication of this increase nutrient supply would be that the vegetation that develops in low litter

treatments would be species of more fertile soils. There was no significant correlation between score on axis 1,

the axis constrained on bracken litter cover and Ellenberg N value (Hill et al. 1999), but there were a significant

number of species present with Ellenberg values greater than that of Calluna (N=2) (Fig. 3).

Discussion

Bracken is an important late-successional invasive species that causes problems for land managers who wish to

restore early-successional communities with a high conservation interest (Pakeman and Marrs 1992, Marrs et

al. 2000). Whilst it has been appreciated for some time that the deep litter layer produced by bracken impedes

re-establishment of many species (Lowday and Marrs 1992), there have been few studies of the impact of

bracken control strategies on litter dynamics and nutrient pools. This study is the first to assess how successful

bracken control treatments are in reducing bracken litter, and their effects on litter turnover, nutrient pools and

species diversity.

Two major findings of this study were derived from the distribution of nutrients within the bracken soil

system. First, there was evidence that for some elements at least there are substantial pools in the recent

standing litter that have not been resorbed into the rhizome. Moreover, the concentrations of the cations reduces

as the new standing litter moves into the older compacted form, suggests large losses\fluxes through leaching.

Second, a major finding of this study was that large amounts of nutrients and especially P, Ca and Mg were

stored in the rhizomes as a proportion of the total amount in the system. This result implies that bracken

preferentially sequesters these nutrients within the rhizome system, where a large pool is developed. The large

amount of Ca and Mg found in the tissues was very surprising and may reflect an evolved mechanism to

survive in very acidic, nutrient-poor conditions.

13

Impact of control strategies on bracken litter mass, nutrient cycling and turnover 1 2 3

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All bracken control treatments impinged on the distribution of litter and nutrients. However, of those control

strategies assessed, cutting twice/yr was most effective in reducing bracken litter. This was not surprising

considering that previous studies on a variety of bracken infested sites have indicated that this treatment tends

to be the most successful at reducing frond productivity by depleting rhizome carbohydrate and nutrient

reserves (Le Duc et al. 2000, Marrs et al. 1998a). However, unique to this study was the evidence that some

treatments could impact on the bracken litter decomposition rate itself, refuting the hypothesis that bracken

litter inputs and standing crop are maintained proportionately as treatment reduces frond mass. We accept that

the litter turnover indices used provide only a crude assessment of decomposition rates (Swift et al. 1979) and

they do not consider the contribution of continued frond senescence to the litter inputs through the season.

Frond senescence begins in the lowermost pinnae shortly after peak frond biomass in late July/early August,

and continues until complete frond death at the time of the first autumn frost (Pitman 1989). Some litter input is

also likely as a result of frond death and damage during the summer, and through physical damage from grazers

or harsh weather. While these unmeasured inputs are likely to be small in comparison with measured inputs,

further work is needed to quantify them. Moreover, no measurements were made of the decomposition rates of

fractions other than the standing bracken fronds, and given the development of non-bracken vegetation in the

treated plots, this is a major weakness, In addition, we only sampled at one point and differences inferred from

untreated plots; further work to derive a time course of change would be valuable. However, the constraint on

studying these, and other processes, is that these experiments were set up for other purposes, and there is a limit

to the damage that we can inflict on them by additional experimentations that involves destructive sampling.

Nevertheless, the result that cutting twice/yr significantly increased turnover rate relative to all other

treatments and the measurements of nutrient fluxes both indicates that this treatment is effective in accelerating

litter turnover, a transfer of nutrients between bracken litter pools and those of other species, and implies much

faster nutrient cycling. Deep stands of bracken litter lock large quantities of nutrients out of effective

circulation, and her we have shown that large amounts can be released, indeed release from bracken pools were

greater than uptake by developing vegetation and their litter. For the most successful treatment, relatively large

amounts of nutrients were released from bracken litter. In absolute terms, bracken management released C and

N but in proportionate terms very large amounts of Ca and Mg were released. This may have substantial

impacts on the vegetation change with a move to eutrophication, and the long-term sustainability of the site, as

these elements are very susceptible to leaching. There is also the intriguing possibility that management to

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provide environmental\conservation benefits will have a C cost, which is outside the spirit of the Kyoto

Protocol. Here we had a net potential release of 1010 g C m-2 over 7 years or 144 g C m-2 y-1. This must be

viewed in context of the C sink in a north Pennine catchment of 15.4 g C m-2 y-1 ( Worrall et al. 2003), and the

predicted sink of 170 reducing to 70 g C m-2 y-1 within 50 years (Karjalainen et al. 2002). A more detailed

knowledge of C fluxes in conservation management treatments to control bracken, and their change through

time, are clearly needed.

An important issue was that there were few treatment effects on litter quality in terms of nutrient

decomposition; all treatments effects were brought about through the effect of the size of the litter pool. This

implies that the comminution of the litter through the passage and operation of the machinery (Pakeman et al.

2000b) is the over-riding factor controlling the decomposition of the bracken litter at this site, operating by

exposing a greater surface area to decomposing organisms Swift et al 1979). We hypothesised that there could

be important interactions with the resource quality of the litter input, which may be affected by treatment. We

might have expected that in untreated bracken the senesced fronds would have a lower nutrient concentration as

a result of enhanced nutrient translocation to the rhizome and would be added to the litter in autumn when

temperatures are cooler. In cut plots, at least a fraction of the standing fronds would have at best a limited

withdrawal of carbohydrates and nutrients into the rhizomes at the time of cutting, and they would be added to

the litter in mid summer when temperatures are higher. We would have expected these combinations of litter

quality and temperatures to influence turnover (Hobbie 1996, Anderson and Hetherington 1999; Hector et al.

2000). However, we found no evidence of resource change in the bracken litter pools of the various treatment.

The grazing sub-treatment had no significant effect on any of the litter pools. This was not surprising

because the sheep stocking levels at Hordron Edge are low in line with Environmentally Sensitive Area

prescriptions. Higher grazing pressures might have increased litter turnover through trampling acting as a low

level continuous cutting treatment, and through additional nutrient cycling via urine and faeces (Wardle et al.

2002).

Relationship between species diversity/composition and above-ground bracken components

The negative relationships between bracken litter variables and the vegetation that has colonised in 7 years,

confirms that persistent deep bracken litter is a major constraint on vegetation establishment following bracken

control (Lowday and Marrs 1992, Mitchell et al. 1997, 1999, Marrs et al. 2000). However, the results also

indicate that reducing bracken litter alone is not sufficient to promote diverse vegetation. The ordination biplots

illustrated, however, that although reducing litter is influential in restoring high diversity, a low grazing

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pressure was also needed. Nutrients released during treatment may also help to promote species of more fertile

soils. Reduced bracken litter led to increases in Galium saxatile, an herb intolerant of grazing and occurring on

infertile soils (Pakeman et al. 2000b). In the unglazed vegetation with low bracken litter, D. flexuosa

dominated, suggesting that fertility was increased in these plots (Diemont and Heil 1984). Low level sheep

grazing had a positive effect on diversity, increasing the cover of a large number of species typical of acid

grassland, probably because moderate grazing created colonisation gaps and decreased shade by limiting the

growth of tall invasive herbs, such as Chamerion angustifolium (Bakker 1998).

Litter depth was identified as the main environmental variable influencing species diversity, and this

suggests that litter is more important than summer frond biomass. This tentative result confirms the conclusions

of a field validation of a model designed to predict the outcome of bracken control treatments on revegetation

(REBRA) (Paterson et al. 2000). Paterson et al. (2000). suggested that the principal reason for poor vegetation

recovery following bracken control is that the slowly decomposing litter provided few opportunities for

seedling germination and subsequent establishment.

Relationship between species diversity and bracken litter turnover

The positive relationship between K2 and species density, and the finding that cutting twice yr-1 resulted in the

greatest biomass of non-bracken vegetation and fastest turnover indices, indicates that there may be positive

feedback between bracken litter decomposition and increasing diversity. Although some studies have found

decomposition to be insensitive to changes in species diversity (Blair et al. 1990, Rustad, 1994), others have

demonstrated an increased turnover rate with increasing diversity due to synergistic non-additive effects of litter

mixing (Salamanca et al. 1998, Bardgett and Shine 1999, Anderson and Hetherington 1999, Hector et al. 2000,

Wardle et al. 2002). For instance, Anderson and Hetherington (1999) found that the mass loss of 1:1 mixtures

of Calluna and bracken litter was 10% greater over the same time period than that of litter of either species on

its own, and in Hector et al’s. (2000) study on acid grassland, mass loss of litter of individual species within

diverse mixtures was greater than that expected based on their losses when alone. However, considering that no

correlations were found between K1 and any measure of species diversity, the results from this study here are

inconclusive. This may result partly from the crude nature of the methodology, the over-riding effect of

comminution within this initially mono-specific litter layer, but it may also be a consequence of the complex

nature of the potential synergistic interactions. Translocation of both nutrients and inhibitory compounds

between different species’ litters can occur by diffusion or fungal hyphae (McTiernan et al. 1997) and, while

movement of nutrients could ameliorate nutrient limitation in the decomposer community, movement of

16

inhibitory compounds could slow this effect (Hector et al. 2000). Hector et al. (2000) proposed this as a reason

why their results indicated that the actual mixture of species involved was probably more important than the

level of diversity alone in influencing decomposition rates. The toxic compounds found in bracken, which make

its fronds unpalatable to most grazers, may presumably also make its litter unpalatable to many decomposers

(Marrs et al. 2000).

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The potential mechanisms whereby cutting and grazing can impact on bracken litter abundance and the

establishment of other species are summarised in Fig. 3; this illustrates the complex nature of the possible

interactions involved. For instance, several possible routes are presented whereby cutting can cause a reduction

in litter abundance. Frond production, and therefore litter input, may be decreased through weakening of

rhizomes via reduced photosynthate translocation, or by reduced shading of the understorey or increasing the

nutrients, and increasing competition from other vegetation. Grazing may enhance decomposition by

encouraging an increase in species diversity or by trampling, mimicking the effects of cutting on turnover

(Wardle et al., 2002). Further experimental work is needed to separate these complex interactions.

Acknowledgements - We thank DEFRA for supporting this long-term project, Neil Taylor and Jeremy Archdale

for permission to work on their land and Dr S Paterson for assistance with the field work.

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FIGURE LEGENDS 1

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Fig. 1 Relationship between species number and (a) uncompacted bracken litter mass, (b) bracken litter

turnover rate based on minimum possible input; the linear regression and variation accounted for by

the regression (r2) and significance are shown; * = P < 0.05, ** = P < 0.01; *** = P < 0.001.

Fig. 2 RDA biplot of the most abundant species and explanatory variables from: (a) Analysis 1, using the

2000 floristic data with the selected significant environmental variables, and (b) Analysis 2, using the

1994-2000 floristic data and constrained on litter cover as axis 1 and all other experimental treatments

and time factored out. Bracken litter is also included as a pseudo-species. Codes for species: Agro cap

= Agrostis capillaris; Agro cas = Agrostis castellana; Agro vin = Agrostis vinealis; Brac lit = bracken

litter; Brac rut = Brachythecium rutabulum; Call vul = Calluna vulgaris; Camp int = Campylopus

introflexus; Camp pyr = Campylopus pyriformis; Care pil = Carex pilulifera; Cham ang = Chamerion

angustifolium; Desc fle = Deschampsia flexuosa; Dicr sco = Dicranum scoparium; Eric tet = Erica

tetralix; Fest ovi = Festuca ovina agg; Gali sax = Galium saxatile; Holc mol = Holcus mollis; Hypn jut

= Hypnum jutlandicum; Loph bid = Lophocolea bidentata; Luzu cam = Luzula campestris; Luzu pil =

Luzula pilosa; Nard str = Nardus stricta; Pleu sch = Pleurozium schreberi; Poly for = Polytrichum

formosum; Pote ere = Potentilla erecta; Pter aqu = Pteridium aquilinum; Rhyt squ = Rhytidiadelphus

squarrosus; Vacc myr = Vaccinium myrtyllus; Vacc vit = Vaccinium vitis-idaea. In (a) grazing

treatments (nominal variables) are shown as the centroids and bracken litter depth as a vector. In (b)

Group 1 consists of Agrostis vinealis, Carex pilulifera, Erica tetralix, Ptilidium ciliare and Vaccinium

vitis-idaea.

Fig. 3 Frequency distribution of species that colonized the experimental site between 1993-2000 based on

Ellenberg N values (Hill et al., 1999).

Fig. 4 Potential mechanisms through which cutting and grazing can influence bracken litter abundance.

Mechanisms for which there is evidence in the results of this study are represented by solid lines.

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Table 1 The effects of the bracken control treatments on bracken litter and frond biomass, non-bracken vegetation\litter and litter turnover indices after seven years. Mean values are given in two forms; arithmetic means in bold for ease of interpretation, and transformed means (loge x+1) on which the analyses of variance were performed. For the latter the LSD (P <0.05), F-ratio and significance are presented (ns = no significance; * = P < 0.05; ** = P < 0.01; *** = P < 0.001). Means not significantly different from each other are denoted with the same letter.

Treatments LSD (P < 0.05)

F5,10 P

Variable

Untreated Asulam Asulam + Cut

Cut + asulam

Cut 1/yr Cut 2/yr

Litter mass Uncompacted bracken litter

382 5.841 a

157 4.058 b

369 5.568 a

317 5.850 a

86 4.058 b

11 1.806 c

1.132

8.94

**

Compacted bracken litter

1642 7.004 a

589 4.610 b

1445 6.807 a

224 5.018 b

63 3.828 bc

15 2.401 c

1.698

6.11

**

Total bracken litter

2024 7.321 a

746 5.027 b

1814 7.237 a

541 6.083 ab

149 4.891 b

27 2.777 c

2.028

7.08

**

Non-bracken vegetation\litter

143 3.328 ab

749 5.619 bc

186 2.688 a

747 6.433 c

781 6.538 c

1010 6.881 c

2.853

3.93

*

Bracken litter turnover indices

K1 (maximum possible litter input)

0.31 0.329 a

0.43 0.251 a

0.54 0.405 ab

0.53 0.408 ab

1.66 0.872 b

5.40 1.725 c

0.508

12.18

***

K2 (minimum possible litter input)

0.26 0.227 a

0.31 0.256 a

0.31 0.261 a

0.62 0.477 a

0.69 0.503 a

5.40 1.725 b

0.481 14.27 ***

Frond mass August 2000 499

6.123 a

191 3.726 ab

671 6.428 a

201 5.271 ab

162 4.794 b

112 3.667 b

2.389

2.37

ns

Table 2. Correlations between bracken, non-bracken vegetation\litter variables and turnover indices. Kendall’s rank correlation (tau b) coefficients are presented

where significance (P < 0.05) was observed; ** = P < 0.01 *** = P < 0.001.

Variable Uncompacted brackenlitter mass (g m-2)

Compacted bracken litter mass (g m-2)

Total bracken litter dry mass (g m-2)

Non-bracken vegetation mass (g m-2)

Litter turnover index (K1)

-0.54 ***

-0.65 ***

-0.65 ***

0.55 ***

Litter turnover index (K2)

-0.42 ***

-0.74 ***

-0.62 ***

0.56 ***

Non-bracken vegetation\litter biomass (g m-2)

-0.53 ***

-0.61 ***

-0.63 ***

-

June frond mass (g m-2)

0.41 ***

0.43 ***

0.45 ***

-0.35 **

August frond mass (g m-2) 0.53***

0.50 ***

0.55 ***

-0.36 **

23

Table 3. Nutrient distribution in the bracken control\ moorland management experiment at Hodron Edge, Derbyshire after seven years treatment: (a) the soil and litter compartments, and (b) the rhizomes. Data are presented as concentrations (C and N = %, other elements = µg g-1) and on an area basis (g m-2, to 15 cm depth). Transformed (loge+1) mean values ± SE (n=36) are presented pooled across all treatments with the arithmetic means in bold. Significance denoted as: - = P > 0.05, * = P < 0.05. ** P < 0.01. *** = P < 0.001. (a) Litter and soil

Soil profile Aoo Ao A1

Element Standing bracken litter

Compacted bracken litter

Non-bracken vegetation\litter

Concentration

C 44.8 3.663±0.150

45.6 3.758±0.109

41.1 3.506±0.179

40.4 3.713±0.023

20.5 3.017±0.056

4.0 1.571±0.042

N 1.5 0.889±0.046

1.8 1.012±0.037

1.9 1.035±0.057

1.8 1.027±0.016

0.7 0.527±0.028

0.2 0.180±0.009

P 531 5.946±0.248

553 6.011±0.196

1125 6.396±0.339

1010 6.874±0.051

843 6.662±0.064

48.2 6.112±0.064

K 5568 8.051±0.346

2453 7.424±0.241

5100 7.803±0.404

2749 7.646±0.165

4341 8.226±0.099

6585 8.760±0.043

Ca 3795 7.735±0.329

292 5.447±0.176

197 4.659±0.303

2264 7.679±0.053

936 6.725±0.098

135 4.716±0.108

Mg 1202 6.673±0.281

459 5.956±0.176

459 5.650±0.293

416 5.980±0.057

435 6.008±0.061

305 5.610±0.085

Na 240 5.158±0.221

144 4.711±0.162

102 4.045±0.283

458 6.090±0.049

734 6.570±0.043

770 6.333±0.029

Area

C 105.1 3.894±0.276

312.2 4.246±0.339

270.6 4.570±0.373

2191 7.571±0.116

2861 7.849±0.077

3234 8.020±0.059

N 3.7 1.219±0.136

12.6 1.634±0.231

13.6 2.113±0.202

97.7 4.482±0.106

9936 4.483±0.084

1.8 5.022±0.065

P 0.1 0.112±0.017

0.6 0.307±0.087

0.8 0.478±0.066

5.5 1.787±0.076

11.6 2.438±0.075

39.3 3.621±0.065

K 1.3 0.681±0.096

1.8 0.641±0.136

3.5 1.180±0.138

14.7 2.495±0.142

61.6 3.927±0.115

549.7 6.244±0.059

Ca 0.9 0.528±0.074

0.2 0.127±0.030

0.1 0.118±0.020

12.4 2.474±0.092

13.2 2.505±0.098

11.2 2.316±0.103

Mg 0.3 0.211±0.030

0.3 0.223±0.056

0.3 0.240±0.031

2.2 1.115±0.052

6.2 1.871±0.076

25.9 3.138±0.084

Na 0.1 0.05±0.008

0.1 0.082±0.024

0.1 0.065±0.012

2.5 1.195±0.059

10.4 2.353±0.058

64.0 4.130±0.051

(b) Element Concentration Area Total rhizome Total expressed

as % of amount in entire profile

Long shoots Short shoots t Long shoots Short shoots t C 42.6

3.776±0.005 44.2 3.812±0.004

4.0***

435 5.969±0.085

156 4.449±0.237 14.3***

591 6.276±0.083

5.7

N 1.2 0.795±0.034

1.2 0.800±0.028

0.1

13.2 2.485±0.101

4.4 1.378±0.136

4.7***

17.6 2.769±0.095

3.9

P 8860 8.916±0.120

6668 8.533±0.155

1.4

9.1 2.101±0.113

2.2 0.906±0.116

10.3***

11.3 2.238±0.103

15.1

K 55525 10.809±0.097

71566 10.838±0.148

0.1

56.0 3.851±0.127

18.7 2.464±0.192

4.3***

74.7 4.147±0.118

10.3

Ca 8128 8.730±0.130

27265 10.06±0.087

6.1***

8.1 1.965±0.120

9.9 1.902±0.170

3.4***

18.0 2.668±0.113

30.0

Mg 27265 9.646±0.089

8620 8.677±0.107

4.9***

17.8 2.744±0.115

2.1 0.967±0.102

12.0***

19.9 2.877±0.107

29.3

Na 658 6.081±0.158

470 5.833±0.161

0.8

0.6 0.432±0.053

0.2 0.147±0.026

6.0***

0.8 0.526±0.057

0.9

24

Table 4. The effects of bracken control treatments on the nutrients in the three litter components (g m-2) in the bracken experiments at Hodron Edge, Derbyshire after seven years treatment. Transformed (loge+1, n=6) means and LSD (P<0.05) values are presented with arithmetic means in bold; means significantly different from each other are denoted by different letters. Significance denoted as: - = P > 0.05, * = P < 0.05. ** P < 0.01. *** = P < 0.001. Compartment Untreated Asulam

+ Cut Cut + Asulam

Asulam Cut 1/yr Cut 2/yr LSD (P<0.05)

F5,10 P

Standing bracken litter

Mass 382 5.841a

369 5.850a

316 5.568a

57 4.058b

86 4.245b

11 1.805c

1.650

8.94

**

C 183 5.103a

176 5.109a

152 4.830ab

74 3.444bc

46 3.523bc

5 1.354

1.480

9.62

**

N 8.5 2.065a

4.9 1.725ab

4.6 1.601b

2.6 1.003bc

1.3 0.782cd

0.2 0.137d

0.784

8.14

**

P 0.20 0.177a

0.20 0.185a

0.18 0.156a

0.10 0.100a

0.05 0.048b

0.01 0.006b

0.107

4.69

*

K 2.8 1.228a

2.2 1.052a

1.8 0.944ab

0.9 0.536bc

0.3 0.257c

0.1 0.069c

0.615

4.96

*

Ca 1.91 1.020a

1.34 0.827a

1.32 0.758a

0.42 0.308b

0.24 0.219b

0.04 0.035b

0.287

18.51

***

Mg 0.39 0.322a

0.37 0.312a

0.45 0.348a

0.22 0.187ab

0.09 0.081b

0.02 0.016b

0.172

6.47

**

Na 0.08 0.079a

0.08 0.077a

0.07 0.081a

0.03 0.032a

0.02 0.023b

0.003 0.003b

0.053

4.14

*

Compacted bracken litter

Mass 1642 7.004a

1446 6.807ab

224 5.018abc

90 4.614bcd

63 3.828cd

15 2.401d

2.247

6.11

**

C 756 6.268a

674 6.073a

111 4.295ab

295 3.971b

30 3.121bc

7 1.746c

2.097

6.78

**

N 35.4 3.200a

25.1 2.860a

4.7 1.465b

9.1 1.380b

1.1 0.667b

0.3 0.232b

1.324

7.87

**

P 1.90 0.818

0.97 0.584

0.09 0.080

0.74 0.330

0.02 0.144

0.01 0.062

0.681

2.4

-

K 4.0 1.249a

4.7 1.537a

0.4 0.292b

1.8 0.601b

0.1 0.127b

0.04 0.036

0.831

5.53

*

Ca 0.31 0.258a

0.45 0.346a

0.07 0.068b

0.07 0.068b

0.02 0.020b

0.01 0.006b

0.163

7.34

**

Mg 0.72 0.475

0.71 0.485

0.10 0.095

0.42 0.242

0.03 0.031

0.01 0.009

0.378

3.24

-

Na 0.20 0.157

0.18 0.163

0.03 0.029

0.2 0.135

0.01 0.009

0.002 0.002

0.171

2.00

-

Non-bracken vegetation\litter

Mass 143 3.328a

186 2.688a

746 6.462b

749 5.619b

781 6.538b

1009 6.880b

2.028

3.93

*

C 64 2.795a

85 2.234a

330 5.608b

343 4.965b

351 5.740b

449 6.075b

2.464

4.40

*

N 3.5 1.009a

4.3 0.861a

16.9 2.682b

13.7 2.318b

17.0 2.794b

19.9 3.012b

1.169

6.42

**

P 0.17 0.141a

0.22 0.167a

1.03 0.662b

1.28 0.696b

0.79 0.523b

1.00 0.679b

0.443

3.48

*

K 0.7 0.429

1.1 0.494

4.0 1.512

7.2 1.716

3.5 1.354

4.6 1.575

1.030

3.03

-

Ca 0.03 0.030a

0.01 0.012a

0.22 0.190b

0.12 0.119b

0.18 0.154b

0.22 0.202b

0.075

11.61

***

Mg 0.07 0.066

0.07 0.061

0.41 0.333

0.37 0.303

0.37 0.306

0.46 0.368

0.174

6.24

**

Na 0.02 0.017a

0.02 0.017a

0.11 0.105b

0.08 0.075b

0.11 0.103b

0.07 0.070b

0.057

4.63

*

25

Table 5. The significant effects of applied treatments in the bracken control\ moorland management experiment at Hodron Edge, Derbyshire; bracken control treatment on (a) rhizome mass (g m-2) and nitrogen concentration on mass basis (%); and (b) nutrient content on an area basis. Transformed (loge+1) means and LSD (P<0.05) values are presented with arithmetic means in bold; means significantly different from each other are denoted by different letters. Significance denoted as: - = P > 0.05, * = P < 0.05. ** P < 0.01. *** = P < 0.001. (a) n=6 Mass compartment\element

Untreated Asulam + Cut

Cut + Asulam,

Asulam Cut 1/yr

Cut 2/yr

LSD (P<0.05)

F5,10 P

Mass – long shoots 1278 7.151 a

1170 6.999 a

1166 6.994 a

1050 6.823 a

959 6.854 a

497 6.100 b

0646

6.56

**

Mass – short shoots 573 6.297

400 4.855

174 4.515

305 4.841

346 5.729

300 5.196

2.199

1.80

-

Mass – total 1851 7.521

1570 7.266

1340 7.136

1355 7.047

1305 7.162

796 6.570

0.914

2.33

-

N - concentration short shoots

1.6 0.968 a

1.4 0.869 ab

1.1 0.725 c

1.3 0.805 bc

1.1 0.721 c

1.1 0.713 c

0.137

4.20

*

(b) n=6 Mass compartment\element

Untreated Asulam + Cut

Cut + Asulam,

Asulam Cut 1/yr

Cut 2/yr

LSD (P<0.05)

F5,10 P

Long shoots C 555

6.318a 505 6.157a

504 6.157a

436 5.973a

399 5.952a

212 5.259b

0.640

6.74

**

N 21.4 3.073a

17.1 2.801ab

11.0 2.423bc

13.8 2.568bc

9.6 2.249cd

6.0 1.795d

0.593

11.14

***

K 57.3 3.973a

62.0 4.089a

58.4 3.966a

68.9 4.106a

62.0 4.112a

27.0 2.856b

0.991

4.88

*

Ca 10.6 2.290a

6.6 1.928ab

9.0 2.145ab

8.6 1.986ab

9.3 2.104ab

5.0 1.334b

0.786

3.56

*

Mg 24.4 3.168a

17.6 2.884a

22.0 3.001a

13.7 2.575ab

21.5 2.964a

7.8 1.866b

0.654

10.34

**

26

Table 6. Nutrient fluxes (g m-2) from vegetation where bracken control treatments have been applied at Hodron Edge, Derbyshire. Arithmetic means ± SE (n=6) are presented. Fluxes were estimated as the difference between the treated plots and its comparator untreated one. A positive value indicates a release of nutrients from the compartment and a negative one indicated uptake relative to the untreated control. The values for the treatments which effected the greatest flux rates (Cut 2/yr) are also expressed as a percentage of the total nutrient stock in the entire profile Compartment Element Bracken control treatment Asulam +

Cut Cut + Asulam,

Asulam Cut 1/yr Cut 2/yr Cut 2/yr fluxes expressed as % of amount in entire profile

Rhizomes C 129.5±151.9 232.4±94.1 246.1±160.6 262.6±46.3 469.5±58.1 4.6 N 8.4±5.0 18.0±3.9 12.3±5.0 17.6±2.2 21.9±3.5 4.8 P -1.4±1.9 1.1±4.8 -1.7±6.1 4.0±1.6 7.4±.2.5 9.8

K 8.4±24.2 25.8±20.0 13.9±35.4 8.5±19.4 50.3±16.0 6.9 Ca 8.7±5.9 12.3±3.9 9.4±6.4 -0.7±10.4 11.6±4.9 19.3 Mg 7.8±3.7 4.2±4.0 12.2±6.1 4.0±7.8 17.9±4.9 26.3 Na 0.2±0.4 0.5±0.3 0.3±0.5 -0.4±0.5 0.4±0.2 0.5

C 87.9±341.5 674.2±279.6 568.4±251.2 867.5±281.8 925.9±278.4 8.9 N 13.8±16.0 34.5±15.0 32.2±15.0 41.5±15.0 43.4±14.8 9.5 P 1.0±1.1 1.9±1.0 1.3±0.6 2.1±1.0 2.1±1.0 2.8

Bracken litter (both fractions)

K -0.1±3.0 4.5±2.6 4.1±3.0 6.3±2.4 6.6±2.3 0.3 Ca 0.4±0.4 0.8±0.8 1.7±0.6 1.9±0.5 2.2±0.5 3.7 Mg 0.03±0.4 0.6±0.4 0.5±0.3 1.0±0.3 1.1±0.3 3.2 Na -0.001±0.1 0.2±0.1 0.04±0.1 0.2±0.1 0.3±0.1 0.4

C -20.6±77.7 -266.5±76.1 -277.9±107.1 -286.8±54.1 -385.4±56.1 -4.0 N -0.8±4.4 -13.4±4.6 -10.2±5.2 -13.5±2.0 -16.3±3.5 -3.6 P -0.04±0.2 -0.9±0.3 -1.12±0.6 -0.6±0.2 -0.8±0.1 -1.1

Non-bracken vegetation and litter

K -0.4±0.9 -3.3±0.9 -6.4±3.0 -2.7±0.9 -3.8±1.1 -0.5 Ca 0.02±0.02 -0.2±0.1 -0.1±0.1 -0.1±0.1 -0.2±0.03 -0.3 Mg 0.01±0.07 -0.3±0.1 -0.3±0.1 -0.3±0.1 -0.4±0.1 -0.6 Na 0±0.02 -0.1±0.03 -0.1±0.04 -0.1±0.04 -0.1±0.02 -0.1

27

Fig 1

28

(a)

-1.0 +1.0

Depth

Grazed

Ungrazed

Brac lit

Gali sax

Pter aquFest ovi

Agro capCamp int

Hypn jut

Rhyt squ

Agro cas

Dicr sco

Desc fle

Pote ere

Agro vin

Eric tet

Vacc vitLuzu pil

Brac rut

Luzu camCamp pyr

Call vul

Pleu schPoly for

Cham ang

Care pil

Axis 2

Axis 1

-1.0

+1.0 (b)

Fig 2

29

7654321

15

10

5

0

Freq

uenc

y

N value

Fig 3

30

Releases nutrients

Reduces shade

Reduces rhizome reserves

Reduces frond

production

Increases competition

Aids establishment

of other vegetation

Reduces litter

Enhances decomposition

Increases species diversity

Grazing Cutting

31Fig 4